Intravascular stent

ABSTRACT

The invention is directed to an expandable stent for implanting in a body lumen, such as a coronary artery, peripheral artery, or other body lumen. The invention provides for an intravascular stent having a plurality of cylindrical rings connected by undulating links. The stent has a high degree of flexibility in the longitudinal direction, yet has adequate vessel wall coverage and radial strength sufficient to hold open an artery or other body lumen. The stent can be compressed or crimped onto a catheter to a very low profile since the peaks that are adjacent the curved portion of the undulating link are shorter than other peaks in the same cylindrical ring to prevent overlap yet still achieve a very low profile, tightly crimped stent onto a catheter.

BACKGROUND OF THE INVENTION

[0001] The invention relates to vascular repair devices, and inparticular intravascular stents, which are adapted to be implanted intoa patient's body lumen, such as a blood vessel or coronary artery, tomaintain the patency thereof. Stents are particularly useful in thetreatment of atherosclerotic stenosis in arteries and blood vessels.

[0002] Stents are generally tubular-shaped devices which function tohold open a segment of a blood vessel or other body lumen such as acoronary artery. They also are suitable for use to support and hold backa dissected arterial lining that can occlude the fluid passageway. Atpresent, there are numerous commercial stents being marketed throughoutthe world. For example, the prior art stents depicted in FIGS. 1-5 havemultiplex cylindrical rings connected by one or more undulating links.While some of these stents are flexible and have the appropriate radialrigidity needed to hold open a vessel or artery, there typically is atradeoff between flexibility and radial strength and the ability totightly compress or crimp the stent onto a catheter so that it does notmove relative to the catheter or dislodge prematurely prior tocontrolled implantation in a vessel.

[0003] What has been needed and heretofore unavailable is a stent whichhas a high degree of flexibility so that it can be advanced throughtortuous passageways and can be readily expanded, and yet have themechanical strength to hold open the body lumen or artery into which itis implanted and provide adequate vessel wall coverage. The presentinvention satisfies this need. That is, the stent of the presentinvention has a high degree of compressibility to secure it on thecatheter and provide a low profile and a high degree of flexibilitymaking it possible to advance the stent easily through tortuousarteries, yet the stent has sufficient radial rigidity so that it canhold open an artery or other blood vessel, or tack up a dissected liningand provide adequate vessel wall coverage.

SUMMARY OF THE INVENTION

[0004] The present invention is directed to an intravascular stent thathas a pattern or configuration that permits the stent to be tightlycompressed or crimped onto a catheter to provide an extremely lowprofile and to prevent relative movement between the stent and thecatheter. The stent also is highly flexible along its longitudinal axisto facilitate delivery through tortuous body lumens, but which is stiffand stable enough radially in its expanded condition to maintain thepatency of a body lumen such as an artery when the stent is implantedtherein.

[0005] The stent of the present invention generally includes a pluralityof cylindrical rings that are interconnected to form the stent. Thestent typically is mounted on a balloon catheter if it is balloonexpandable or mounted on or in a catheter without a balloon if it isself-expanding.

[0006] Each of the cylindrical rings making up the stent have a proximalend and a distal end and a cylindrical plane defined by a cylindricalouter wall surface that extends circumferentially between the proximalend and the distal end of the cylindrical ring. Generally thecylindrical rings have a serpentine or undulating shape which includesat least one U-shaped element, and typically each ring has more than oneU-shaped element. The cylindrical rings are interconnected by at leastone undulating link which attaches one cylindrical ring to an adjacentcylindrical ring. The undulating links are highly flexible and allow thestent to be highly flexible along its longitudinal axis. At least someof the undulating links have a curved portion that extends transverse tothe stent longitudinal axis for a predetermined distance that coincideswith one of the U-shaped elements. More specifically, the curved portionextends in a transverse manner such that it would intersect with thecorresponding U-shaped element, however, the corresponding U-shapedelement is shorter in length than other U-shaped elements in the samering. Thus, when the stent is compressed or crimped onto the catheter,the curved portions do not overlap or intersect with the adjacentU-shaped element since that element is shorter in length than similarU-shaped elements in the particular ring. In this manner, the stent canbe compressed or crimped to a much tighter or smaller diameter onto thecatheter which permits low profile delivery as well as a tight grippingforce on the catheter to reduce the likelihood of movement between thestent and the catheter during delivery and prior to implanting the stentin the vessel.

[0007] The undulating links may take various configurations but ingeneral have an undulating or serpentine shape. The undulating links caninclude bends connected by substantially straight portions wherein thesubstantially straight portions are substantially perpendicular to thestent longitudinal axis.

[0008] Not only do the undulating links that interconnect thecylindrical rings provide flexibility to the stent, but the positioningof the links also enhances the flexibility by allowing uniformflexibility when the stent is bent in any direction along itslongitudinal axis. Uniform flexibility along the stent derives in partfrom the links of one ring being circumferentially offset from the linksin an adjacent ring. Further, the cylindrical rings are configured toprovide flexibility to the stent in that portions of the rings can flexor bend and tip outwardly as the stent is delivered through a tortuousvessel.

[0009] The cylindrical rings typically are formed of a plurality ofpeaks and valleys, where the valleys of one cylindrical ring arecircumferentially offset from the valleys of an adjacent cylindricalring. In this configuration, at least one undulating link attaches eachcylindrical ring to an adjacent cylindrical ring so that at least aportion of the undulating links is positioned within one of the valleysand it attaches the valley to an adjacent peak.

[0010] While the cylindrical rings and undulating links generally arenot separate structures, they have been conveniently referred to asrings and links for ease of identification. Further, the cylindricalrings can be thought of as comprising a series of U's, W's and Y-shapedstructures in a repeating pattern. Again, while the cylindrical ringsare not divided up or segmented into U's, W's and Y's, the pattern ofthe cylindrical rings resemble such configuration. The U's, W's and Y'spromote flexibility in the stent primarily by flexing and by tippingradially outwardly as the stent is delivered through a tortuous vessel.

[0011] The undulating links are positioned so that the curved portion ofthe link is outside the curved part of the W-shaped portion. Since thecurved portion does not substantially expand (if at all) when the stentis expanded, it will continue to provide good vessel wall coverage evenas the curved part of the W-shaped portion spreads apart as the stent isexpanded. The curved portion of the link extends in a directiontransverse to the stent longitudinal axis for a distance that positionsit adjacent and proximal to the peak of a U-shaped element. TheseU-shaped elements have struts that are shorter than the struts of theother U-shaped elements in the same cylindrical ring so that as thestent is compressed the curved portion of the link does not overlap theadjacent U-shaped element.

[0012] In one embodiment, the W-shaped portion has a first and secondradius at its base where the first radius is greater than the secondradius so that the first radius expands more easily than the secondradius when the stent is expanded. The first radius corresponds with asecond peak (U-shaped member) which is shorter than the other peaks inthe ring. The second peak has shorter struts than the struts of theother peaks and as a result expands more slowly when the stent expands.Thus, faster expansion rate of the first radius of the W-shaped portionhas a tendency to compensate for the slower expansion rate of theadjacent shorter second peak to provide overall uniform expansion of thestent. Also, the shorter second peak can have a greater radius than thelonger first peaks, again to provide different expansion rates to obtainmore uniform stent expansion.

[0013] In another embodiment, each ring has nine peaks, three each offirst, second, and third peaks. The third peak has the longest struts,the second peak the shortest struts, and the first peak has intermediatelength struts. In order to obtain uniform stent expansion, the radius ofthe peaks is inversely proportional to the strut length. The shortersecond peak with the shortest struts has the biggest peak radius, thefirst peak has an intermediate radius, and the third peak with thelongest struts has the smallest peak radius.

[0014] The number and location of undulating links that interconnectadjacent cylindrical rings can be varied as the application requires.Since the undulating links typically do not expand when the cylindricalrings of the stent expand radially outwardly, the links are free tocontinue to provide flexibility and to also provide a scaffoldingfunction to assist in holding open the artery. Importantly, the additionor removal of the undulating links has very little impact on the overalllongitudinal flexibility of the stent. Each undulating link isconfigured so that it promotes flexibility whereas some prior artconnectors actually reduce flexibility of the stent.

[0015] The cylindrical rings of the stent are plastically deformed whenexpanded when the stent is made from a metal that is balloon expandable.Typically, the balloon-expandable stent is made from a stainless steelalloy or similar material.

[0016] Similarly, the cylindrical rings of the stent expand radiallyoutwardly when the stent is formed from superelastic alloys, such asnickel-titanium (NiTi) alloys. In the case of superelastic alloys, thestent expands upon application of a temperature change or when a stressis relieved, as in the case of a pseudoelastic phase change.

[0017] Because of the undulating configuration of the links, the stenthas a high degree of flexibility along the stent axis, which reduces thetendency of stent fishscaling. Stent fishscaling can occur when thestent is bent and portions of the stent project outward when the stentis in the unexpanded condition. The present invention undulating linksreduce the likelihood of fishscaling.

[0018] Further, because of the positioning of the links, and the factthat the links do not expand or stretch when the stent is radiallyexpanded, the overall length of the stent is substantially the same inthe unexpanded and expanded configurations. In other words, the stentwill not substantially shorten upon expansion.

[0019] The stent may be formed from a tube by laser cutting the patternof cylindrical rings and undulating links in the tube. The stent alsomay be formed by laser cutting a flat metal sheet in the pattern of thecylindrical rings and links, and then rolling the pattern into the shapeof the tubular stent and providing a longitudinal weld to form thestent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is an elevational view, partially in section, of a priorart stent mounted on a rapid-exchange delivery catheter and positionedwithin an artery.

[0021]FIG. 2 is an elevational view, partially in section, similar tothat shown in FIG. 1 wherein the prior art stent is expanded within theartery, so that the stent embeds within the arterial wall.

[0022]FIG. 3 is an elevational view, partially in section, showing theexpanded prior art stent implanted within the artery after withdrawal ofthe rapid-exchange delivery catheter.

[0023]FIG. 4 is a plan view of a flattened prior art stent whichillustrates the pattern of the stent shown in FIGS. 1-3.

[0024]FIG. 5 is a side view of the prior art stent of FIG. 4 in acylindrical configuration and in an unexpanded state.

[0025]FIG. 6A is a plan view of a flattened stent of one embodiment ofthe invention which illustrates the pattern of the rings and links.

[0026]FIG. 6B is a partial plan view of the stent of FIG. 6A which hasbeen expanded to approximately 3.0 mm inside diameter.

[0027]FIG. 6C is a plan view of a portion of the stent of FIG. 6A rolledinto a cylindrical configuration and tightly crimped so that the variousstent struts are either in close contact or contacting each other.

[0028]FIG. 7A is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0029]FIG. 7B is a partial plan view of the stent of FIG. 7A which hasbeen expanded to approximately 4.0 mm inside diameter.

[0030]FIG. 7C is a portion of the stent of FIG. 7A that is illustratedin a cylindrical configuration and is tightly crimped or compressed.

[0031]FIG. 8A is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0032]FIG. 8B is a plan view of the flattened stent of FIG. 8A where therings and links have been crimped or tightly compressed.

[0033]FIG. 8C is a plan view of a portion of the flattened stent of FIG.8A illustrating the relationship of the U-shaped member to theundulating link prior to crimping the stent.

[0034]FIG. 9A is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0035]FIG. 9B is a plan view of the flattened stent of FIG. 9A where therings and links have been crimped or tightly compressed.

[0036]FIG. 9C is a portion of the flattened stent of FIG. 9Aillustrating the relationship of the shortened U-shaped member and theundulating portion of the link when the stent is in a partially crimpedor compressed configuration.

[0037]FIG. 10A is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0038]FIG. 10B is a plan view of the flattened stent of FIG. 10A in acrimped or compressed configuration.

[0039]FIG. 10C is a partial plan view of the flattened stent of FIG. 10Adepicting the relationship between the shortened U-shaped member and theundulating portion of the link when the stent is partially crimped orcompressed.

[0040]FIG. 11A is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0041]FIG. 11B is a plan view of the stent of FIG. 11A depicting therings and links in a crimped or compressed configuration.

[0042]FIG. 11C is a partial plan view of the flattened stent of FIG. 11Adepicting the relationship between the shortened U-shaped member and theundulating portion of the link when the stent is partially linked orcompressed.

[0043]FIG. 12 is a plan view of the stent of FIG. 10A rolled into acylindrical configuration and in a crimped or compressed configuration.

[0044]FIG. 13 is a plan view of the stent of FIG. 10A in a cylindricalconfiguration and illustrating the rings and links in an expandedconfiguration.

[0045]FIG. 14 is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of rings and links.

[0046]FIG. 15 is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and linkswhere each of the rings has nine peaks.

[0047]FIG. 16 is a plan view of a flattened stent of another embodimentof the invention which illustrates the pattern of the rings and links.

[0048]FIG. 17 is a plan view of a flattened stent depicting anotherembodiment of the invention which illustrates the pattern of rings andlinks.

[0049]FIG. 18 is an enlarged partial perspective view of a portion of apeak and associated struts depicting variable thickness struts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The present invention stent improves on existing stents byproviding a longitudinally flexible stent having a uniquely designedpattern and novel interconnecting members. In addition to providinglongitudinal flexibility, the stent of the present invention alsoprovides radial rigidity and a high degree of scaffolding of a vesselwall, such as a coronary artery. The design of the highly flexibleinterconnecting members and their placement relative to an adjacentU-shaped member provides for a tightly compressed stent onto a catheterwhile maintaining a high degree of flexibility during delivery.

[0051] Turning to the drawings, FIG. 1 depicts a prior art stent 10mounted on a conventional catheter assembly 12 which is used to deliverthe stent and implant it in a body lumen, such as a coronary artery,peripheral artery, or other vessel or lumen within the body. Thecatheter assembly includes a catheter shaft 13 which has a proximal end14 and a distal end 16. The catheter assembly is configured to advancethrough the patient's vascular system by advancing over a guide wire byany of the well known methods of an over the wire system (not shown) ora well known rapid exchange catheter system, such as the one shown inFIG. 1.

[0052] Catheter assembly 12 as depicted in FIG. 1 is of the well knownrapid exchange type which includes an RX port 20 where the guide wire 18will exit the catheter. The distal end of the guide wire 18 exits thecatheter distal end 16 so that the catheter advances along the guidewire on a section of the catheter between the RX port 20 and thecatheter distal end 16. As is known in the art, the guide wire lumenwhich receives the guide wire is sized for receiving various diameterguide wires to suit a particular application. The stent is mounted onthe expandable member 22 (balloon) and is crimped tightly thereon sothat the stent and expandable member present a low profile diameter fordelivery through the arteries.

[0053] As shown in FIG. 1, a partial cross-section of an artery 24 isshown with a small amount of plaque that has been previously treated byan angioplasty or other repair procedure. Stent 10 is used to repair adiseased or damaged arterial wall which may include the plaque 26 asshown in FIG. 1, or a dissection, or a flap which are sometimes found inthe coronary arteries, peripheral arteries and other vessels.

[0054] In a typical procedure to implant prior art stent 10, the guidewire 18 is advanced through the patient's vascular system by well knownmethods so that the distal end of the guide wire is advanced past theplaque or diseased area 26. Prior to implanting the stent, thecardiologist may wish to perform an angioplasty procedure or otherprocedure (i.e., atherectomy) in order to open the vessel and remodelthe diseased area. Thereafter, the stent delivery catheter assembly 12is advanced over the guide wire so that the stent is positioned in thetarget area. The expandable member or balloon 22 is inflated by wellknown means so that it expands radially outwardly and in turn expandsthe stent radially outwardly until the stent is apposed to the vesselwall. The expandable member is then deflated and the catheter withdrawnfrom the patient's vascular system. The guide wire typically is left inthe lumen for post-dilatation procedures, if any, and subsequently iswithdrawn from the patient's vascular system. As depicted in FIGS. 2 and3, the balloon is fully inflated with the prior art stent expanded andpressed against the vessel wall, and in FIG. 3, the implanted stentremains in the vessel after the balloon has been deflated and thecatheter assembly and guide wire have been withdrawn from the patient.

[0055] The prior art stent 10 serves to hold open the artery after thecatheter is withdrawn, as illustrated by FIG. 3. Due to the formation ofthe stent from an elongated tubular member, the undulating components ofthe stent are relatively flat in transverse cross-section, so that whenthe stent is expanded, it is pressed into the wall of the artery and asa result does not interfere with the blood flow through the artery. Thestent is pressed into the wall of the artery and will eventually becovered with endothelial cell growth which further minimizes blood flowinterference. The undulating portion of the stent provides good tackingcharacteristics to prevent stent movement within the artery.Furthermore, the closely spaced cylindrical elements at regularintervals provide uniform support for the wall of the artery, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of the artery, as illustrated in FIGS. 2 and3.

[0056] One of the problems associated with some prior art stents such asthe one shown in FIG. 4, is the ability to more tightly crimp orcompress the stent 10 onto the balloon portion of the catheter. Forexample, the undulating portion 27 of the links 28 of the prior artstent in FIG. 4 are positioned between two struts 29A/29B so that as thestent is tightly crimped or compressed onto the balloon portion of thecatheter, the struts can only come so close to the undulating portionbefore contact is made. Preferably, the undulating portion and theadjacent struts should not overlap, therefore the undulating portion ofthe link limits the amount of the crimping or compression of eachcylindrical ring onto the balloon portion of the catheter. The presentinvention solves this problem and allows for a tightly compressed orcrimped stent onto the catheter.

[0057] In keeping with the present invention, FIGS. 6-16 depict stent 30in various configurations. Referring to FIG. 6A, for example stent 30 isshown in a flattened condition so that the pattern can be clearlyviewed, even though the stent is in a cylindrical form in use, such asshown in FIG. 6C. The stent is typically formed from a tubular member,however, it can be formed from a flat sheet such as shown in FIG. 6A androlled into a cylindrical configuration as shown in FIG. 6C.

[0058] As shown in FIGS. 6-16, stent 30 is made up of a plurality ofcylindrical rings 40 which extend circumferentially around the stentwhen it is in a tubular form (see FIGS. 6C, 7C, 8B, 9B, 10B, 11B and12). The stent has a delivery diameter 42 as shown in FIG. 12, and animplanted diameter 44 as shown in FIG. 13. Each cylindrical ring 40 hasa cylindrical ring proximal end 46 and a cylindrical ring distal end 48.Typically, since the stent is laser cut from a tube there are nodiscreet parts such as the described cylindrical rings and links.However, it is beneficial for identification and reference to variousparts to refer to the cylindrical rings and links and other parts of thestent as follows.

[0059] Each cylindrical ring 40 defines a cylindrical plane 50 which isa plane defined by the proximal and distal ends 46,48 of the ring andthe circumferential extent as the cylindrical ring travels around thecylinder. Each cylindrical ring includes cylindrical outer wall surface52 which defines the outermost surface of the stent, and cylindricalinner wall surface 53 which defines the innermost surface of the stent.Cylindrical plane 50 follows the cylindrical outer wall surface.

[0060] In keeping with the invention, undulating link 54 is positionedwithin cylindrical plane 50. The undulating links connect onecylindrical ring 30 to an adjacent cylindrical ring 30 and contribute tothe overall longitudinal flexibility to the stent due to their uniqueconstruction. The flexibility of the undulating links derives in partfrom curved portion 56 connected to straight portions 58 wherein thestraight portions are substantially perpendicular to the longitudinalaxis of the stent. Thus, as the stent is being delivered through atortuous vessel, such as a coronary artery, the curved portions 56 andstraight portions 58 of the undulating links will permit the stent toflex in the longitudinal direction which substantially enhances deliveryof the stent to the target site. The number of bends and straightportions in a link can be increased or decreased from that shown, toachieve differing flexibility constructions. With the straight portionsbeing substantially perpendicular to the stent longitudinal axis, theundulating link acts much like a hinge at the curved portion to provideflexibility. A straight link that is parallel to the stent axistypically is not flexible and does not add to the flexibility of thestent.

[0061] Referring to FIGS. 6-16, the stent 30 can be described moreparticularly as having a plurality of first peaks 60, second peaks 61,and valleys 62. Although the stent is not divided into separateelements, for ease of discussion references to peaks and valleys isappropriate. The number of peaks and valleys can vary in number for eachring depending upon the application. Thus, for example, if the stent isto be implanted in a coronary artery, a lesser number of peaks andvalleys are required than if the stent is implanted in a peripheralartery, which has a larger diameter than a coronary artery. As can beseen for example in FIG. 6A, peaks 60,61 are in phase 63, meaning thatthe peaks 60,61 point in the same direction and are substantiallyaligned along the longitudinal axis of the stent. It may be desirableunder certain circumstances to position the peaks so that they are outof phase (not shown), that is, the peaks of one ring would becircumferentially offset from the peaks of an adjacent ring so that theapex of adjacent peaks pointed toward each other. As shown in FIGS.6-16, the peaks are circumferentially offset 64 from the valleys andfrom the undulating link 54. Positioning the peaks, valleys, andundulating links in this manner, provides a stent having uniformexpansion capabilities, high radial strength, a high degree offlexibility, and sufficient wall coverage to support the vessel.

[0062] In keeping with the invention, and as shown in FIGS. 6-16, eachof the cylindrical rings has a plurality of first peaks 60 which havefirst struts 66 attached to a first apex 67. The first struts can beeither curved or straight depending upon the particular application. Thecylindrical rings also have second peaks 61 which have second struts 68attached to a second apex 69. Again, the second struts can be eithercurved or straight depending upon the particular application.Importantly, the length of the second struts 68 are shorter than thelength of the first struts 66. As can be seen in FIGS. 6C, 7C, 8B, 9A,9B, 9C, 10A, 10B, 10C, 11A, 11B, 11C and 12, when the stent is in acrimped condition, or a partially crimped condition, the first strutsand second struts respectively will be closer to each other when thestent is compressed or crimped onto the balloon or expandable member ofthe catheter. The crimping or compressing process, however, also movesthe undulating link 54 along with its curved portion 56 closer to thesecond peak. In order to allow the stent to be more tightly crimped ontothe balloon portion of the catheter, and to avoid overlapping betweenthe undulating link and the second peak, the second struts 68 areshorter than the first struts 66, thus avoiding any overlapping contactbetween the curved portion of the undulating link and the second peak.The various stent struts, curved portions, links, and peaks and valleysmay contact each other when the stent is crimped or compressed, butoverlapping is an undesirable feature.

[0063] More particularly, in order to more tightly crimp or compress thecylindrical rings 40 of the stent 30, the undulating link 54 is tightlycrimped or compressed into contact with, or near contact with, secondpeak 61. As can be seen, for example, in FIG. 6C, curved portion 56 andstraight portions 58 are in close relation to second peak 61 and areeither in contact (not shown) or near contact with second apex 69. Thecurved portion is proximal to the second peak and the various struts ineach of the rings are tightly compressed to be in contact or nearcontact with each other. For example, first struts 56 and second struts58 as well as arm 76 of the undulating link all are in close contact, orcontact with each other in order to provide a very low profile, tightlycrimped stent onto the balloon portion of the catheter. Likewise, if thestent is formed of a self-expanding material such as nickel-titanium,the stent will similarly be tightly crimped and positioned within asheath or within the catheter for delivery in the vascular system.Importantly, the curved portion and the straight portions of theundulating link are positioned relative to the second peak to allow thestent to be tightly crimped as described.

[0064] As can be seen in FIGS. 6-16, there are slight variations indiffering embodiments of the present invention. For example, the firststruts 66 and the second struts 68 of the stent depicted in FIGS. 6A-6C,are curved and have several bends along their length. In contrast, asshown in FIGS. 9A-9C, the first struts and second struts aresubstantially straight. Whether the various struts are substantiallystraight or have slight bends is a matter of choice to suit a particularapplication.

[0065] Referring to FIGS. 6-16, the stent 30 of the invention also canbe described as having cylindrical rings formed of U-shaped portions 70,Y-shaped portions 72, and W-shaped portions 74. Again, while the stentis generally laser cut from a tube and it typically has no discreetparts, for ease of identification the stent of the invention also can bereferred to as having U-, Y-, and W-shaped portions. The U-shapedportions have no supporting structure attached thereto. The Y-shapedportions, at their base, or apex, have arm 76 extending therefrom whichis attached to undulating link 54. The W portion has at its base orcurve portion an arm 78 which attaches at the other end of theundulating link. The length of the arms attaching the links to the ringscan vary.

[0066] Due to the intricate patterns as disclosed in FIGS. 6-13, therate of expansion of the various portions of the stent, including theU-shaped portion 70, the Y-shaped portion 72, and the W-shaped portion74, can vary. Accordingly, one aspect of the invention provides fordifferent radii of curvature at various points so that the stent willexpand evenly and uniformly. Thus, first radius 71 which correspondswith first peak 60 has a smaller radius of curvature than does secondradius 72 which corresponds with second peak 61. Generally, the longerthe struts associated with a peak, the more easily that portion of thestent will expand, so that a smaller radius is associated with peakshaving longer struts. Likewise, for peaks, such as second peak 61, whichhas struts 68 that are shorter than the struts 66 of first peak 60, hasa greater radius of curvature which will expand more easily in order tocompensate for the stiffer bending moments created by the shorter struts68.

[0067] Also referring to FIGS. 6-13, the radius of curvature of thevarious portions of the W-shaped portion also varies to provide uniformstent expansion. Since the second peak 61 and its associated struts 68have a tendency to expand more slowly as the stent is expanded, agreater radius of a curvature is provided in the adjacent part of theW-shaped portion 74. Thus, third radius 75 of the W-shaped portion 74 isgreater than the fourth radius 77 in the W-shaped portion. The thirdradius 75 is adjacent to second peak 61 which has a tendency to expandmore slowly, while fourth radius 77 is adjacent the first peak 60 whichhas a tendency to expand more easily. By varying the radii of curvaturein the W-shaped portion, the stent will expand more evenly andcompensate for the varying rates of expansion of adjacent portions in acylindrical ring.

[0068] It is also a design feature that more or fewer undulating links54 will be positioned between adjacent cylindrical rings 40. Further, inorder to increase stent stability, straight links 80, as shown in FIG.11A, in addition to undulating links 54, connect adjacent cylindricalrings. The straight links will provide stability and assist inpreventing stent foreshortening, as do the undulating links. Further,the straight links may provide more rigidity in a localized area, suchas at the stent ends, such that it may be desirable to incorporate morestraight links between the cylindrical rings at the stent ends than inthe center of the stent.

[0069] In an alternative embodiment as shown in FIG. 14, stent 30 isdesigned to provide good vessel wall coverage and greater expandabilitysince each cylindrical ring 40 has eight peaks 90. Generally, the morepeaks in a cylindrical ring that has an undulating pattern, the greaterthe expansion capabilities of that particular ring. Further, the stentof FIG. 14 has a greater number of links 54 than in some of the otherstent patterns. In this embodiment, there are four undulating links 54between adjacent rings so that the stent has uniform flexibility andmaintains sufficient vessel wall coverage.

[0070] Referring to FIG. 15, an alternative embodiment of stent 30 isshown in which each cylindrical ring 40 has nine peaks 90. As with thestent pattern depicted in FIG. 14, the stent pattern of FIG. 15 iscapable of expanding to a greater diameter due to the greater number ofpeaks 90 and yet maintain sufficient vessel wall coverage. In thisembodiment, the first peak 60 and second peak 61 are substantially thesame as previously described with respect to the stent patterns depictedin FIGS. 6-13. In this embodiment, however, a third peak 92 has a pairof third struts 93 and a third apex 94. Third peak 92 has third struts93 that are longer than the first struts 66 and the second struts 68 ofthe first peak 60 and the second peak 61 respectively. As with the otherembodiments, the struts 66 of the first peak 60 are longer than thesecond struts 68 of second peak 61. Further, in order to provide moreuniform expansion of the stent, the third radius 95 of the third peak 92is smaller than the first radius 71 of first peak 60. Likewise, aspreviously described, first radius 71 is smaller than second radius 73of second peak 61. Generally speaking, the radius of curvature of thepeaks are inversely proportional to the length of the struts so that thelonger the struts the smaller the radius of curvature relative toshorter struts with a greater radius of curvature. As the stent expands,the peak having a greater radius of curvature will expand more easilythan those having a smaller radius of curvature, thus, compensating forthe length of the struts in which the peaks having shorter struts have atendency to expand more slowly than peaks having longer struts and whichhave moment arms that bend more easily.

[0071] Referring to FIG. 16, the stent 30 is similar to the otherembodiments except that the radius of curvature of all of the peaks andvalleys are somewhat larger in order to make it easier to laser cut thestent pattern from a tubular member or from a flat sheet.

[0072] Turning to FIG. 17, in an alternative embodiment, the stent 30includes a pattern that does not have a so-called W-shaped portion. Inthis embodiment, the undulating link 54 is substantially proximal to thesecond peak 61, with a slight portion of the undulating link 54 beingcircumferentially adjacent to the second peak. The first peak 60 stillhas struts 66 that are longer than struts 68 of second peak 61 so thatthe stent of this embodiment functions in substantially the same manneras that described for the other stent embodiments.

[0073] In one aspect of the invention, after stent 30 is implanted in acoronary artery, or other vessel, because of its novel design, thecylindrical rings 40 have the ability to flex radially as the vesselpulsates when blood pumps through it. Likewise, because of the novel andunique design of undulating links 54, as the vessel moves and pulsatesfrom the pumping blood, the stent can flex longitudinally. The radialand longitudinal flexing of the stent reduces the likelihood that thestent will cause injury to the intima of a coronary artery, which alsomay have a tendency to reduce the likelihood of restenosis.

[0074] In another aspect of the invention, the stent 30 is formed sothat the various struts of the cylindrical rings, including the U-shapedportions 70, Y-shaped portions 72, W-shaped portions 74, and theundulating links 54, all can be formed so that each has a variablethickness along the stent length. For example, the undulating link, andits associated arms 76,78 may be thicker at one end (arm 76) than at theother end of the link (arm 78). Further, first struts 66 and secondstruts 68 may vary in thickness (radial thickness) along their length inorder to create variable flexibility in the rings. As shown in FIG. 16,first peak 60 has first struts 66 that have radial thick portion 80 inthe middle of the struts and radial thin portion 82 near the ends of thestruts. As another example, the rings at for example the proximal end ofthe stent may be thicker radially than the rings in the center of thestent. A variable thickness stent that would benefit from the presentinvention is described and disclosed in U.S. Ser. No. 09/343,962 filedJun. 30, 1999 and entitled VARIABLE THICKNESS STENT AND METHOD OFMANUFACTURE THEREOF, which is incorporated herein in its entirety byreference thereto. A variable thickness stent would benefit from theflexible nature of the present invention stent and still be crimped to avery low profile delivery diameter due to the novel relationship betweenthe second peak 61 and the undulating link 54.

[0075] The stent 30 of the present invention can be mounted on a ballooncatheter similar to that shown in the prior art device in FIG. 1. Thestent is tightly compressed or crimped onto the balloon portion of thecatheter and remains tightly crimped onto the balloon during deliverythrough the patient's vascular system. When the balloon is expanded, thestent expands radially outwardly into contact with the body lumen, forexample, a coronary artery. When the balloon portion of the catheter isdeflated, the catheter system is withdrawn from the patient and thestent remains implanted in the artery. Similarly, if the stent of thepresent invention is made from a self-expanding metal alloy, such asnickel-titanium or the like, the stent may be compressed or crimped ontoa catheter and a sheath (not shown) is placed over the stent to hold itin place until the stent is ready to be implanted in the patient. Suchsheaths are well known in the art. Further, such a self-expanding stentmay be compressed or crimped to a delivery diameter and placed within acatheter. Once the stent has been positioned within the artery, it ispushed out of the catheter or the catheter is withdrawn The stentdiameter is very small, so the tubing from which it is made mustnecessarily also have a small diameter. Typically the stent has an outerdiameter on the order of about 0.06 inch in the unexpanded condition,the same outer diameter of the tubing from which it is made, and can beexpanded to an outer diameter of 0.1 inch or more. The wall thickness ofthe tubing is about 0.003 inch.

[0076] The tubing is mounted in a rotatable collet fixture of amachine-controlled apparatus for positioning the tubing relative to alaser. According to machine-encoded instructions, the tubing is rotatedand moved longitudinally relative to the laser which is also machinecontrolled. The laser selectively removes the material from the tubingby ablation and a pattern is cut into the tube. The tube is thereforecut into the discrete pattern of the finished stent.

[0077] The process of cutting a pattern for the stent into the tubing isautomated except for loading and unloading the length of tubing. In oneexample, a CNC-opposing collet fixture for axial rotation of the lengthof tubing is used in conjunction with a CNC X/Y table to move the lengthof tubing axially relatively to a machine-controlled laser. The entirespace between collets can be patterned using the CO₂ laser set-up of theforegoing example. The program for control of the apparatus is dependenton the particular configuration used and the pattern to be ablated inthe coating.

[0078] Cutting a fine structure (0.005 to 0.001 inch web width) requiresminimal heat input and the ability to manipulate the tube withprecision. It is also necessary to support the tube yet not allow thestent structure to distort during the cutting operation. In order tosuccessfully achieve the desired end results, the entire system must beconfigured very carefully. The tubes are made typically of stainlesssteel with an outside diameter in the range of about 0.060 inch to 0.070inch and a wall thickness in the range of about 0.002 inch to 0.005inch. These tubes are fixtured under a laser and positioned utilizing aCNC to generate a very intricate and precise pattern. Due to the thinwall and the small geometry of the stent pattern (about 0.0035 inchtypical web width), it is necessary to have very precise control of thelaser, its power level, the focused spot size, and the precisepositioning of the laser cutting path. proximally and the stent held inplace until it exits the catheter and self-expands into contact with thewall of the artery. Balloon catheters and catheters for deliveringself-expanding stents are well known in the art.

[0079] The stent 30 of the present invention can be made in many ways.One method of making the stent is to cut a thin-walled tubular member,such as stainless steel tubing to remove portions of the tubing in thedesired pattern for the stent, leaving relatively untouched the portionsof the metallic tubing which are to form the stent. The stent also canbe made from other metal alloys such as tantalum, nickel-titanium,cobalt-chromium, titanium, shape memory and superelastic alloys, and thenobel metals such as gold or platinum. In accordance with the invention,it is preferred to cut the tubing in the desired pattern by means of amachine-controlled laser as is well known in the art.

[0080] The tubing may be made of suitable biocompatible material such asstainless steel. The stainless steel tube may be Alloy type: 316L SS,Special Chemistry per ASTM F138-92 or ASTM F139-92 grade 2. SpecialChemistry of type 316L per ASTM F138-92 or ASTM F139-92 Stainless Steelfor Surgical Implants in weight percent. Carbon (C) 0.03% max. Manganese(Mn) 2.00% max. Phosphorous (P) 0.025% max. Sulphur (S) 0.010% max.Silicon (Si) 0.75% max. Chromium (Cr) 17.00-19.00% Nickel (Ni)13.00-15.50% Molybdenum (Mo) 2.00-3.00% Nitrogen (N) 0.10% max. Copper(Cu) 0.50% max. Iron (Fe) Balance

[0081] In order to minimize the heat input into the stent structure,which prevents thermal distortion, uncontrolled burn out of the metal,and metallurgical damage due to excessive heat, and thereby produce asmooth debris free cut, a Q-switched Nd-YAG, typically available fromQuantronix of Hauppauge, N.Y., that is frequency doubled to produce agreen beam at 532 nanometers is utilized. Q-switching produces veryshort pulses (<100 nS) of high peak powers (kilowatts), low energy perpulse (≦3 mJ), at high pulse rates (up to 40 kHz). The frequencydoubling of the beam from 1.06 microns to 0.532 microns allows the beamto be focused to a spot size that is 2 times smaller, thereforeincreasing the power density by a factor of 4 times. With all of theseparameters, it is possible to make smooth, narrow cuts in the stainlesstubes in very fine geometries without damaging the narrow struts thatmake up to stent structure. Hence, the system of the present inventionmakes it possible to adjust the laser parameters to cut narrow kerfwidth which will minimize the heat input into the material.

[0082] The positioning of the tubular structure requires the use ofprecision CNC equipment such as that manufactured and sold by AnoradCorporation. In addition, a unique rotary mechanism has been providedthat allows the computer program to be written as if the pattern werebeing cut from a flat sheet. This allows both circular and linearinterpolation to be utilized in programming. Since the finishedstructure of the stent is very small, a precision drive mechanism isrequired that supports and drives both ends of the tubular structure asit is cut. Since both ends are driven, they must be aligned andprecisely synchronized, otherwise the stent structure would twist anddistort as it is being cut.

[0083] The optical system which expands the original laser beam,delivers the beam through a viewing head and focuses the beam onto thesurface of the tube, incorporates a coaxial gas jet and nozzle thathelps to remove debris from the kerf and cools the region where the beaminteracts with the material as the beam cuts and vaporizes the metal. Itis also necessary to block the beam as it cuts through the top surfaceof the tube and prevent the beam, along with the molten metal and debrisfrom the cut, from impinging on the opposite surface of the tube.

[0084] In addition to the laser and the CNC positioning equipment, theoptical delivery system includes a beam expander to increase the laserbeam diameter, a circular polarizer, typically in the form of a quarterwave plate, to eliminate polarization effects in metal cutting,provisions for a spatial filter, a binocular viewing head and focusinglens, and a coaxial gas jet that provides for the introduction of a gasstream that surrounds the focused beam and is directed along the beamaxis. The coaxial gas jet nozzle (0.018 inch I.D.) is centered aroundthe focused beam with approximately 0.010 inch between the tip of thenozzle and the tubing. The jet is pressurized with oxygen at 20 psi andis directed at the tube with the focused laser beam exiting the tip ofthe nozzle (0.018 inch dia.). The oxygen reacts with the metal to assistin the cutting process very similar to oxyacetylene cutting. The focusedlaser beam acts as an ignition source and controls the reaction of theoxygen with the metal. In this manner, it is possible to cut thematerial with a very fine kerf with precision. In order to preventburning by the beam and/or molten slag on the far wall of the tube I.D.,a stainless steel mandrel (approx. 0.034 inch dia.) is placed inside thetube and is allowed to roll on the bottom of the tube as the pattern iscut. This acts as a beam/debris block protecting the far wall I.D.

[0085] Alternatively, this may be accomplished by inserting a secondtube inside the stent tube which has an opening to trap the excessenergy in the beam which is transmitted through the kerf along whichcollecting the debris that is ejected from the laser cut kerf. A vacuumor positive pressure can be placed in this shielding tube to remove thecollection of debris.

[0086] Another technique that could be utilized to remove the debrisfrom the kerf and cool the surrounding material would be to use theinner beam blocking tube as an internal gas jet. By sealing one end ofthe tube and making a small hole in the side and placing it directlyunder the focused laser beam, gas pressure could be applied creating asmall jet that would force the debris out of the laser cut kerf from theinside out. This would eliminate any debris from forming or collectingon the inside of the stent structure. It would place all the debris onthe outside. With the use of special protective coatings, the resultantdebris can be easily removed.

[0087] In most cases, the gas utilized in the jets may be reactive ornon-reactive (inert). In the case of reactive gas, oxygen or compressedair is used. Compressed air is used in this application since it offersmore control of the material removed and reduces the thermal effects ofthe material itself. Inert gas such as argon, helium, or nitrogen can beused to eliminate any oxidation of the cut material. The result is a cutedge with no oxidation, but there is usually a tail of molten materialthat collects along the exit side of the gas jet that must bemechanically or chemically removed after the cutting operation.

[0088] The cutting process utilizing oxygen with the finely focusedgreen beam results in a very narrow kerf(approx. 0.0005 inch) with themolten slag re-solidifying along the cut. This traps the cut out scrapof the pattern requiring further processing. In order to remove the slagdebris from the cut allowing the scrap to be removed from the remainingstent pattern, it is necessary to soak the cut tube in a solution of HClfor approximately 8 minutes at a temperature of approximately 55° C.Before it is soaked, the tube is placed in a bath of alcohol/watersolution and ultrasonically cleaned for approximately 1 minute to removethe loose debris left from the cutting operation. After soaking, thetube is then ultrasonically cleaned in the heated HCl for 1-4 minutesdepending upon the wall thickness. To prevent cracking/breaking of thestruts attached to the material left at the two ends of the stentpattern due to harmonic oscillations induced by the ultrasonic cleaner,a mandrel is placed down the center of the tube during thecleaning/scrap removal process. At completion of this process, the stentstructure are rinsed in water. They are now ready for electropolishing.

[0089] The stents are preferably electrochemically polished in an acidicaqueous solution such as a solution of ELECTRO-GLO#300, sold byELECTRO-GLO Co., Inc. in Chicago, Ill., which is a mixture of sulfuricacid, carboxylic acids, phosphates, corrosion inhibitors and abiodegradable surface active agent. The bath temperature is maintainedat about 110°-135° F. and the current density is about 0.4 to about 1.5amps per in.². Cathode to anode area should be at least about two toone. The stents may be further treated if desired, for example byapplying a biocompatible coating.

[0090] It will be apparent that both focused laser spot size and depthof focus can be controlled by selecting beam diameter and focal lengthfor the focusing lens. It will be apparent that increasing laser beamdiameter, or reducing lens focal length, reduces spot size at the costof depth of field.

[0091] Direct laser cutting produces edges which are essentiallyperpendicular to the axis of the laser cutting beam, in contrast withchemical etching and the like which produce pattern edges which areangled. Hence, the laser cutting process essentially provides strutcross-sections, from cut-to-cut, which are square or rectangular, ratherthan trapezoidal. The struts have generally perpendicular edges formedby the laser cut. The resulting stent structure provides superiorperformance.

[0092] Other methods of forming the stent of the present invention canbe used, such as using different types of lasers; chemical etching;electric discharge machining; laser cutting a flat sheet and rolling itinto a cylinder; and the like, all of which are well known in the art atthis time.

[0093] The stent of the present invention also can be made from metalalloys other than stainless steel, such as shape memory alloys. Shapememory alloys are well known and include, but are not limited to,nickel-titanium and nickel-titanium-vanadium. Any of the shape memoryalloys can be formed into a tube and laser cut in order to form thepattern of the stent of the present invention. As is well known, theshape memory alloys of the stent of the present invention can includethe type having superelastic or thermoelastic martensitictransformation, or display stress-induced martensite. These types ofalloys are well known in the art and need not be further described here.

[0094] Importantly, a stent formed of shape memory alloys, whether thethermoelastic or the stress-induced martensite-type, can be deliveredusing a balloon catheter of the type described herein, or be deliveredvia a catheter without a balloon or a sheath catheter.

[0095] While the invention has been illustrated and described herein, interms of its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other body lumens.Further, particular sizes and dimensions, number of undulations orU-shaped portions per ring, materials used, and the like have beendescribed herein and are provided as examples only. Other modificationsand improvements may be made without departing from the scope of theinvention.

What is claimed:
 1. A flexible intravascular stent for use in a bodylumen, comprising: a plurality of cylindrical rings aligned along acommon longitudinal axis and interconnected to form the stent, eachcylindrical ring having a first delivery diameter and a second implanteddiameter; each cylindrical ring having a plurality of first peaks andsecond peaks, each of the peaks having a height, the second peaks beingshorter than the first peaks; and at least one undulating link attachingeach cylindrical ring to an adjacent cylindrical ring, the undulatinglinks having a curved portion extending transverse to the stentlongitudinal axis toward the second peak, the height of the second peakbeing sized so that as the stent is compressed to the first deliverydiameter, the curved portion is positioned proximal to the second peak.2. The stent of claim 1, wherein at least one undulating link comprisesat least one curved portion connected to a substantially straightportion, the substantially straight portion being substantiallyperpendicular to the stent longitudinal axis.
 3. The stent of claim 2,wherein the substantially straight portion of the at least oneundulating link is perpendicular to the stent longitudinal axis when thestent is in the first delivery diameter configuration.
 4. The stent ofclaim 2, wherein the substantially straight portion of the at least oneundulating link is perpendicular to the stent longitudinal axis when thestent is in the second implanted diameter configuration.
 5. The stent ofclaim 1, wherein at least one of the undulating links comprise aplurality of curved portions.
 6. The stent of claim 1, wherein the firstpeaks of each cylindrical ring are in phase with the first peaks of anadjacent cylindrical ring.
 7. The stent of claim 1, wherein theundulating links are configured to provide flexibility to the stent. 8.The stent of claim 1, wherein the cylindrical rings are configured toprovide flexibility to the stent.
 9. The stent of claim 1, wherein thestent is formed from a tube.
 10. The stent of claim 1, wherein the stentis formed from a flat sheet.
 11. The stent of claim 1, wherein the stentis formed from a metal alloy.
 12. The stent of claim 11, wherein thestent is formed from any of the group of metal alloys consisting ofstainless steel, tantalum, nickel-titanium, cobalt-chromium andtitanium.
 13. The stent of claim 1, wherein the stent is formed from ashape memory alloy.
 14. The stent of claim 13, wherein the stent isformed from the group of shape memory alloys consisting ofnickel-titanium and nickel-titanium-vanadium.
 15. The stent of claim 1,wherein the stent is formed from a superelastic or pseudoelastic metalalloy.
 16. The stent of claim 15, wherein the stent is formed from thegroup of superelastic or pseudoelastic metal alloys consisting ofnickel-titanium and nickel-titanium-vanadium.
 17. The stent of claim 1,wherein at least a portion of the stent has a variable thicknessconfiguration.
 18. The stent of claim 1, wherein at least a portion ofthe first peaks has a variable thickness configuration.
 19. The stent ofclaim 1, wherein at least a portion of the second peaks has a variablethickness configuration.
 20. The stent of claim 1, wherein at least aportion of the undulating links has a variable thickness configuration.21. The stent of claim 1, wherein at least a portion of the cylindricalring has a variable thickness configuration.
 22. The stent of claim 1,wherein the first peaks have a first radius and the second peaks have asecond radius, the second radius being greater than the first radius.23. The stent of claim 1, wherein each cylindrical ring has a pluralityof third peaks, each of the third peaks having a height, the third peaksbeing longer than the first peaks and the second peaks.
 24. The stent ofclaim 1, wherein each of the first peaks has a pair of first struts andeach of the second peaks has a pair of second struts, the first strutsbeing longer than the second struts.
 25. The stent of claim 1, whereineach cylindrical ring has a plurality of third peaks and a pair of thirdstruts associated with the third peaks.
 26. The stent of claim 25,wherein the first peaks have a pair of first struts, the second peakshave a pair of second struts, the third struts being longer than thefirst struts, and the first struts being longer than the second struts.